1. Field of the Invention
The present invention relates to a hollow fiber membrane and a method for manufacturing the same, and more particularly to a through-one-end water collection type hollow fiber membrane having a free end which is not fixed to a header whereby permitting relatively free movement of the membrane when submerged in feed water to be treated.
2. Discussion of the Related Art
A separation method using a membrane has lots of advantages compared to the method based on a phase inversion or heating. Among the advantages is high reliability of water treatment since the water purity required may be easily and stably satisfied by adjusting the size of the pores of a membrane. Further, a membrane can be used with microorganism which is useful for separation process but may be adversely affected by heat.
A membrane for separation includes a flat sheet membrane and a hollow fiber membrane.
A hollow fiber membrane module carries out a separation process using a bundle of hollow fiber membranes. Typically, a hollow fiber membrane module has been widely used in the field of microfiltration and ultrafiltration for obtaining axenic water, drinking water, super pure water, and so on. Recently, however, application of the hollow fiber membrane module is being expanded to include wastewater treatment, solid-liquid separation in a septic tank, removal of suspended solid (SS) from industrial wastewater, filtration of river, filtration of industrial water, and filtration of swimming pool water.
Among such hollow fiber membrane modules is a submerged hollow fiber membrane module which is submerged into a tank of feed water to be treated. Negative pressure is applied to the internal parts of the hollow fiber membranes such that only fluid passes through the walls of the membranes and solids and sludge are rejected and accumulate in the tank. A submerged hollow fiber membrane module is used mainly in the form of a cassette having a plurality of modules combined to a frame. A submerged hollow fiber membrane module is advantageous in that the manufacturing cost is relatively low and that the installation and maintenance cost may be reduced since a facility for circulating fluid is not required.
When a submerged hollow fiber membrane module is used to treat wastewater, the solids in the wastewater fouls the membrane causing the permeability of the membrane to be declined as the wastewater is treated. The solids may be present in the feed water in a variety of forms which contribute to fouling in different ways. To counter the different types of fouling, many different types of cleaning regimens may be required.
Such cleaning may be classified into maintenance cleaning and recovery cleaning according to the cleaning purposes.
The maintenance cleaning is a cleaning performed while the water treatment is carried out by the hollow fiber membrane module or a cleaning performed only for a short time after the water treatment is stopped. The main purpose of the maintenance cleaning is to maintain the permeability of the membranes in good status. The maintenance cleaning is generally carried out by physical cleaning. The most frequently used methods of physical cleaning are backwashing and aeration.
In backwashing, permeation through the membranes is stopped momentarily. Air or water flows through the membranes in a reverse direction to physically push solids off of the membranes. On the other hand, in aeration, bubbles are produced in the tank water below the membranes. As the bubbles rise, they agitate or scrub the membrane and thereby remove the solids while creating an air lift effect and circulation of the tank water to carry the solids away from the membranes.
Based on the water collection type, a submerged hollow fiber membrane module may be classified into a through-both-ends water collection type and a through-one-end water collection type. According to a through-both-ends water collection type, the permeate obtained inside each hollow fiber membrane is collected through both ends of the membranes. On the other hand, the permeate is collected through only one end of each membrane in a through-one-end water collection type.
In case of a through-both-ends water collection type hollow fiber membrane module, two ends of the membrane are fixed to two headers respectively. Each header has a permeate collecting space therein with which the membrane is in fluid communication. When performing maintenance cleaning by means of aeration, upward movement of bubbles from a aeration tube are interrupted by the headers especially when the hollow fiber membrane module is a vertical type. An upper header has the effect of displacing the rising bubbles towards the outside of the membrane bundle. Thus, effective aeration is no longer guaranteed in the upper region of the membrane. As a consequence, relatively severe fouling occurs in the upper region of the membrane bundle.
On the other hand, in case of a through-one-end water collection type hollow fiber membrane module, only one end of the membrane is fixed to a header and the other end, a free end, is free to move. Thus, interruption of water flow caused by rising bubbles emitted from the aeration tube is remarkably reduced, and thus a vertical hollow fiber membrane of through-one-end water collection type may guarantee more effective aeration over the entire length of the membrane than a vertical hollow fiber membrane of through-both-ends water collection type. For this reason, a hollow fiber membrane of through-one-end water collection type has been actively studied.
Since the free ends of the membranes in the through-one-end water collection type hollow fiber membrane module are not fixed to a header, every each of the membranes must be sealed at their free ends. Since the durability of sealing part is relatively weak, the durability of the whole hollow fiber membrane depends on how to seal the free ends of the membranes.
A method of sealing a hollow fiber membrane at its free end comprises coating the free end with a sealant of same polymer, e.g., polyethersulfone (PES), as that of the membrane, and curing the sealant. FIG. 1 shows a cross section of a hollow fiber membrane sealed with this method. As shown in FIG. 1, the sealing part 110 exists only on the external surface of the free end of the hollow fiber membrane 100. Thus, the sealing part 110 is vulnerable and might be easily stripped off from the membrane 100 causing leakage, which requires replacement of the impaired membrane 100 and thus increases the maintenance cost.